Frequency modulated communications system with multiplexed audio channels



5 Shee'ts-Sheet 1 INVENTOR FRANK A. HESTER ATTORNEY CL 22, 1957 F. A.HEsTER FREQUENCY NODULATED COMMUNICATIONS SYSTEM WITH MULTIPLEmn AUDI@CHANNELS Filed Dec. 2l, 1953 Ct 22, 1957 F. A. HEsTER 2,810,782

FREQUENCY MODULATED comUNlcAfrIoNs SYSTEM WITH MULTIPLEED AUDIO CHANNELSFiled Dec. 2l, 1953 5 SheetshSheet 2 INVENTOR FRANK A. HESTER BY @ELATTORNEY Oct. 22, 1957 F. A. HESTER 2,810,782 Y FREQUENCY MODULATEDoom/ruNIcATIoNs SYSTEM WITH MULTIPLEXED AUDIO CHANNELS Filed Deo. 2l,1953 5 Sheets-Sheet 3 .ATTORNEY Oct. 22, 1957 F. A. HESTER FREQUENCYMODULATED comuNcATroNs SYSTEM WITH MULTIPLEXED AUDIO CHANNELS Filed Dec.21, 1953 5 Sheets-Sheet 4- A TTORNEY Oct. 22, 1957 F. A. HEs-rERFREQUENCY MODULATED COMMUNICATIONS SYSTEM WITH MULTIPLEXED AUDIOCHANNELS 5 sheets-sheet 5 Filed Dec. 21. 1953 INVENTOR FRANK A. HESTERBY ATTORNEY United States Patent O FREQUENCY MODULATED COMNIUNICATIONSSYSTEM WITH MULTIPLEXED AUDIO CHAN- NELS Frank A. Hester, New York, N.Y., assignor to Hogan Laboratories, Inc., New York, N. Y., a corporationof New York Application December 21, 1953, Serial N o. 399,353

Claims. (Cl. 1785.6)

The present invention relates to a new system of radio multiplexing, andmore particularly to a signaling system wherein a plurality of signalsare simultaneously transmitted Via a single frequency modulated radiofrequency carrier.

This application is a continuation-impart of application Serial No.84,622, filed March 3l, 1949, now abandoned.

It is a principal object of the present invention to provide apparatusfor simultaneously transmitting a plurality of audio frequency signalson a single frequency modulated radio frequency carrier withoutoccurrence of interference or cross modulation between the signals.

It is a further object to provide a system for frequency modulationtransmission wherein within the standard and conventional 200 megacyclefrequency band allotted to a frequency modulated transmitter stationone, two or more auxiliary channels may be accommodated in addition tothe primary audio frequency channel without lowering the quality of theprimary audio signal below the high standard of excellence currentlyrequired of such stations.

It is a further object of the invention to provide a multiplex systemfor transmitting in one channel a signal of superaudible frequencyamplitude modulated by a subcarrier which subcarrier is amplitudemodulated by an audio frequency signal, and for transmitting in anotherchannel another audio frequency signal, with crossmodulation betweenchannels effectively prevented; and for simultaneously frequencymodulating a radio frequency carrier by the output of both channels.

It is a further object to provide a system of multiplexing plural signalchannels in a radio frequency modulator of conventional type designed totransmit a single audio channel without requiring modification oralteration of the modulator to accommodate the additional channels.

lt is a further object to provide a novel multiplex system for aprimaryV audio signal channel and plural auxiliary signal channels inwhich a conventional frequency modulation receiver receives the primaryaudio signal without interference from the auxiliary signals andauxiliary receivers receive the auxiliary signals without interferencefrom the primary audio signal and without interference between theauxiliary signals.

It is a further object to provide a system for multiplexing facsimileand audio signals in such a manner that neither the facsimile signalsnor the audio signals are inferior in quality or power compared withsimilar signals transmitted separately.

It is a further object to provide an audio-facsimile multiplex systemwhich incorporates standard broadcast frequency-modulation transmittingand receiving equipment and standard facsimile equipment.

Others have attempted to achieve some of the objects above stated withvarying degrees of success. Among the prior systems proposed may bementioned those of E. H. Armstrong as described in Patent 2,104,012 andH. Roder as described in Patent 2,233,183. A basic di'iculty encounteredby all prior art systems was that cross modula- Patented Oct. 22, 1957tion between the several channels was not effectively prevented. Afurther disadvantage encountered was that conventional frequencymodulation receivers designed to receive only the primary audiofrequencies required certain material modilications to avoid receivingthe auxiliary signals.

No claim is made by the present inventor to the broad idea or thefundamental principles involved in simultaneous transmission of primaryaudio and auxiliary signals. However, as the result of exhaustiveanalyses and extensive experiments, certain new features relating tosuch system have been invented which, for the rst time, make possiblethe full attainment of commerciallysuccessful frequency modulationmultiplexing.

In multiplexing primary audio and auxiliary signals according to theinvention, the auxiliary signals are carried by one or more superaudiblecarriers and are combined with a primary audio signal originating assound signals in the frequency range of from 30 to 15,000 cycles persecond. The combined signal is used to frequency modulate a radiofrequency carrier. The systems of the prior art above mentioned employnon-linear circuits such as individual amplifiers at the outputs of theseveral channels which supply the primary audio and superaudible signalsvia a common path to the modulator of the n'ansmitter. At the outputs ofthe several amiiliary channels are produced undesired audiofrequency'subharmonics of the superaudible frequency carrier due toinherent non-, V linearity of the amplifiers and undesired audiofrequency signals as a result of partial demodulation of thesuperaudible frequency carriers, and at the primary audio channel outputare produced undesired superaudible frequency signals which areharmonics of the primary audio signals in the primary channel. All theseundesired signal frequencies which are solely the result ofnon-linearityV of the several circuit outputs are combined in the commonpath to the transmitter modulator with each other and with the desiredaudio and superaudible frequency signals. The resulting interferencebetween the several signals renders satisfactory multiplexingimpossible. The

present invention avoids this troublesome interference by non-linearityof the amplifier which further increases in- Y terference between theseveral signals. The present invention avoids this source ofinterference by providing a common linear transmission path from theoutputs of all channels directly to the modulator of the transmitter.

A particularly important feature of the present invention is that theaudio frequency signals carried by the frequency modulated radiofrequency carrier may be received by a standard type of frequencymodulation receiver without interference from the superaudible frequencysignals also used to modulate the carrier. In order to insure that thereis no perceptible interference between the primary audio frequencysignals and any auxiliary signals in the receiver for the primarysignals, the amplitudes of the several signals modulating the radiofrequency carrier at the transmitter are xed at predetermined relativelevels which effectively minimize such interference.

At the presenttime standard audio programs are being broadcast to thepublic by means of frequency-modulated radio waves in the neighborhoodof to 110 megacycles. One of the advantages of transmission by frequencymodulation is that a high signal-to-noise ratio can be maintained evenin the presence of electrical disturbances of considerableV magnitude.In order to attain this high signal-to- Certain systems of the prior artabove meni Y standardFM audio frequency programs.

noise ratio, radio broadcasting stations have been requiredto provide adegree of modulation such that the transmitted frequencies deviateV 75kilocycles on each side of the center carrier frequency ati 100 percentmodulation.-

At the present time there are a@ great many receivers in the hands ofthe public which Vare capable ofrreceivingfsuchVV interest of thoseengaged in broadcasting high-fidelity EMV programs must not beprejudiced by permitting the mu1ti' plexed transmission of auxiliarysignalsto interfere there- With. Also a commercially. acceptablemultiplexingV systemmustbevoperable without.reductionofquality orV in Yper second and Ygenerallyy originateas-sound signals.v The;

auxiliary signals. above mentioned,.- also refer toaudio fre,-

The present-v Y superaudible frequency subcarrier is so fixed withYrelaiquency. signals in the range of 0-1 0,000-cycles persecond; Y

and; may originate as soundl signals in `afradio1 ortelephone system, asfacsimile signals-derived from graphic; copybya facsimilescanner, asi-acoded message as in telegraph:v

and teletype systems; as pulses'obtainedv int telemeteringand,otherfinstrumentation systems, ,retc.v .j t

.A.preferred; embodiment of the inventionwill berex= plainedfwithlreference toy a, systemfin` which. facsimilepsignals are multiplexedwith the-primary audio signals; This: isint'endedtozbe illustrative ofonly one manner inwhich; the invention-2 maybeused since it-,will` beapparentthati other.Y types ofV signalsmay be multiplexed with;theprimary' audio signals in addition toer insteadJ of facsimile: signals;Other embodiments; ofthe invention'willralso lle-described;

ulated by primary audio frequency signals, which originate as a soundprogram occupying arfrequency range of to 15,000 cycles per second, andthe carrieris simultaneously frequency modulated by facsimile programsignals carried by a modulated superaudible frequency subcarrieroccupying a band about 6,000 cycles per second wide.Theemaximum-Yamplitude of the amplitude'modu- Y lated superaudibleVfrequency"subcarrier` is fixed;V atV a predetermined relative valuewith respect to the maximumV amplitude of the primary audio frequencysignals so that. the radio frequency carrier is modulated in. the` sameratio; that is, the deviation oftherra'dio frequency carrierby the tiontoA Vthe'` deviationcausedby. the; primary.' audio frequency signalsVand withrelation to the. total deviation of the radio frequency carrier,that no interference between the signals is perceptible at theloudspeaker of a standard frequency modulation receiven; Stated.quantitatively the ratio` of theV deviations, ofl the radiok frequencycarrier produced by the several. modulating signals produces aninterfering-signal in a standard-frequency' modulation receiver whichVhas an amplitude at least 60 decibels less thanr the amplitude ofthedesired audio frequency signals.

The receiving system of the presentinvention includes an adapterconnectible to a standard frequency; modulation receiver forthe'selection,Y amplification and recordingof the facsimile programsignals... TheV facsimile signal' ,Ther term; facsimile is'intended todesigDatethe; artfofi' transmission! of graphicmaterialthroughtheamedium ot electrical signals.

tures and/ or test is optically scannedl pointfby-pointand; Y

Graphic material inV the form ofV picreproducing means. is operatedwithout, modicationof the circuitvordisturbance withthefnormalfunctioning: ofV the receiver for reproducing theoriginalsoundprograms. Likewise.A the facsimile: signal;receiving andy reproducing.;

means kkis operatedvwithout interferenceA by theprimary audio-.frequencysignals.V Y

In a., modification; of Vthe. invention;v employing av pri!V maryVaudio'freqnency` signalv chauneland plural auxiliary audio.frequencysignalfchannelsrlhe severalchannels arel provided with.suitablezrlter-s.. at; their outputs? and with.

. a commonlinear"transmission path-tothe frequency modline-.by-line.,thevariations inlightreectedlfrom the: copy beingtranslated'into-electrical Variationsby means of' a.-

photoelectric cell. These electrical variationsy may con sist offrequencies in the range of'l from 0*-33000. cycles.`

persecondandl maybe yursedto amplitude Ymodulate arsubif;

carrier having azfrequencyfof 10,1000 cycles perqsecondiin:

onel channel of a` multiplex systemrembodying; theinven-J- tion.Theresulting doublesidebandmodulated.sulJcarr-ierl Will hen,covera-frequeny,band:,of;from:7,000rtor 13 ,000Y Y cycles per second; andissuitable: for transmission. over.-V

ulatorof the transmitter. mits a subcarrier` of' a'` unique;superaudible; *frequency*V which; is; amplitude; modulated. by audiofrequency sig-V nals. in; that channel. The-maximum'. deviation of theradio-frequency carrier caused by allthemodu'latedsubtelephone-wirelines. Thismodulatedsubcarrienmay then. beheterodyned,fwithasuperaudiblefrequency. subcarrier,

filtered, amplilied and filtered again, thenrcombinedwitl.'V

the filtered audio frequency output oftheprimary;audioA channel ina"linear'Ytransmission'l network. The combinedsignals. aref delivereddirectly toA a frequencyY modulator Vwhere a radiofrequency;carrier.is-modulatedi The modu-Y lated carrierv is:transmitted;or radiatedto a distantA point.4

where thefmodulated carrier, is received by. a conventional- Y frequencymodulation receiyer. In; the'receiver the pri-` mary audio signal isderived; fromk the demcdulated:v carrier. intheusualway..VV ,TheAauxiliary signalsgrwhether facsimileor otherwi'searefdivertedinto akseparatezchan-i.

'nell If. there aremore thanouesetfof auxiliary. signalsp theyQareseparated into their,respectivechannels byrlte'rs:

accordingtoA-thefrequency` bandsfoccupied :bytheir modu-V`H AsuperaJ-Ldiblerwarriors, they original'auxiliar'y signals arelobtained'il The auxiliary facsimile signals-faredelivered-to afacsimile, recorder .whereV theoriginal copy is recreatedlatedsuperaudible carriers; Afterfdemodulation=of= the-f pointbyzpoint and'line-by-line. Suitable'reproducersfare" employed: for: other: typeslof' auxiliary signals:

In :theirst emb'odimentfofithe -invention to be described;

the transmittedfradio-frequency carrierr4 is frequency-mod"-H YcarriersI isixed at.` such a. predeterminedvalue.: that theyinterference, ratiobetweemthe primary andf'auxiliary signals in theoutputfof aistandard.frequency'modulation Y Y receiver is-atleast-V60decibels;l

Bor., a betterunderstanding of therinyentiongereferenceis had. to.) thefollowing descriptiomtaleenineconjunction.

withithe. .appended drawings-wherein:

Fig. l. is a blockdiagram. of-aportiorr of; af. multiplex transmission?system. arrangediuaV accordance; with` the Y teaching ofthis'.invention.

Fig. Z is a-blocktdiagramiofra multiplex receiyinglsystemfor use.inassociation withrthe;transmission..system; of4 Fig., 1Y and1arranged.. in accordance, withethepteaching; of. the invention; toinclude a1 frequency,A modulation reeceiver withranauxiliary;signalreceivingL channeLconnected thereto. e

Fig.,.3 shows a inthe systems of Figs. 1 'and;5. Y

Eig.v4,shows.a circuit diagramfot:V anY- adapter amplifier.

whichmay. be employed inthe receiving. system of Figs. Y.

2 and 7. s

Big5 showsa blockV diagram:v ofra-.portionrof another multiplextransmission. system: arranged in. accordance. YVwith; the teachingofthe invention- V Y Fig. 6 shows achar'acteristic;curveausefulzinfexplaine 'ing.the.invention.. y

Fig. 7. 'shows a'. receivingL system; usablei with the.- syS- l.'

Y Referring now. infgreater. detail `to;the;drawing;ps5` IEig.. l.V .eshows: a. system including a facsimile. scannen 7; which may includeapickup scanneriphotoelectrio.cellFandfarnf plif'erS.: ."Th'e Scanner"derives*Y electrical' 'signals from Each; auxiliary channel` transfl .Y

multiplexing?circuitwhicrn.mayrbe used Y asioffsa Scanned graphic copyhaving a frequency range of zero to 3,000 cycles per second. Thesefacsimile signals are delivered to an amplitude modulator 9. Anoscillator 8 delivers a -ten kilocycle subcarrier signal to themodulator 9. The facsimile signals amplitude modulate the subcarrier inthe modulator to produce a double sideband wave output occupying theaudio frequency band of 7 to 13 kilocycles. The modulator output isdelivered to preamplifier 10. The circuit thus far described correspondswith that disclosed in my prior Patent 2,545,463. The output ofpreamplifier 10 and the output of a 35 kc. oscillator 12 are applied toa balanced or ring'modulator 13. The 35 kc. output of oscillator 12 isamplitude modulated in modulator 13 by the 7 to 13 kc. output of amplier10. The output of the modulator lies between the limits of 22 and 48 kc.and consists of one group of frequencies in the range of 22 to 28 kc.and a mirror image group in the range of 42 to 48 kc. The 35 kc. carrieris suppressed in the modulator. Low pass filter 14 arranged to passsignals up to 35 kc. eliminates the 42 to 48 kc. group and passes the 22to 28 kc. group to amplifier 15. A portion of the signals appearing inamplifier 15 may be passed through amplifier 16 to a metering circuit17. The signal frequencies in the range of from 22 to 28 kc. are appliedfrom amplifier 15 to bandpass lter 18 which passes only the signalfrequencies in the band of from 22 to 28 kc. This signal is then appliedthrough a linear variable attenuator 19 to mixing pad 20. Mixing pad 20is a linear network which with attenuator 19 constitutes a direct lineartransmission path to the frequency modulator and radio frequencytransmitter 24.

The sound program signal which may be in the frequency range of 30 to15,000 cycles per second is applied from a sound source 3 via microphone4 and a preemphasis circuit to an audio amplifier 21. Afterarnplification, the audio signals are applied to low pass lter 23 whichpasses only those frequency components below about 16 kc. to mixing pad20. The combined amplitude modulated superaudible carrier signal and theaudio signals are applied via mixing pad 20 directly to the modulatorand frequency modulation radio transmitter 24 in a common lineartransmission path. A radio frequency carrier frequency modulated by theaudio signals and the amplitude modulated superaudible carrier isradiated from antenna 22 to a receiving system to be described.

It will be noted from Fig. 1 that there is no non-linear common path forthe superaudible frequency facsimile signal and the primary audiosignal, as for example through a common ampiier. Since all commerciallypractical amplifiers are subject to some cross modulation due toinherent non-linearity, the arrangement shown is vastly superior to thedevices of the prior art in preventing interference between thefacsimile signals and the primary audio signals which are both audiofrequency signals. The mixing pad 20, being a purely linear network doesnot introduce non-linear distortion so that cross modulation in thecommon linear transmission path to the frequency modulator between thefacsimile and other audio signals does not occur. Attention is alsoinvited to the reason for the introduction of the low pass filter 23between the audio amplifier 21 and the mixing pad 2t). The secondharmonics of l1 to 14 kc. audio signals fall in the superaudiblefrequency signal range of 22 to 28 kc. and these harmonics are preventedfrom reaching the transmitter modulator 24 because of the presence oflter 23. Interference in the facsimile recording due to these harmonicsis thereby precluded.

The maximum amplitudes of the input signals to the mixing pad 20 fromthe respective channels, that is, the maximum amplitudes of the outputsof the primary audio channel and the auxiliary channel will be fixed ata predetermined ratio in accordance with certain considerations to beexplained below. A satisfactory ratio for the two channel arrangement ofFig. 1 will be about 14 to 1,

so that the primary audio signals cause a maximum deviation of the radiofrequency carrier of about kc., while the auxiliary signal causes adeviation of about 5 kc. making the total deviation kc. Some of thereasons for selecting this ratio of amplitudes are as follows:

In the operation of the system of this invention, with audio frequencyprogram signals occupying the range of 30-15,000 cycles per secondtransmitted in the primary channel, and facsimile program signalsoccupying a range of 0-3,000 cycles carried by a superaudible frequencysubcarrier in an auxiliary channel, the radio frequency carrier isfrequency modulated by the primary audio program to a degree such thatpercent modulation causes a frequency deviation of about 70 kc. on eachAside of the center frequency. The radio frequency carrier is furtherfrequency modulated by the superaudible frequency facsimile program to adegree such that 100 percent modulation causes a frequency deviation ofabout 5 kc. on each side of the center frequency. In other words, thefull standard 75 kc. deviation is used for both the primary audioprogram and multiplexed auxiliary facsimile program with about 5 kc. ofdeviation used for the facsimile program. It will be remembered,according to the principles of frequency modulation that the rate ofchange of frequency of the carrier wave carries the intelligence and theextent or deviation of the frequency change determines the degree ofamplitude of modulation. It has been found that facsimile signaltransmission at modern speeds of 360 lines per minute and 28.1 squareinches per minute can be duplexed with audio signal transmissionemploying about 70 kc. deviation without any mutual interference andwithout requiring a special receiver to exclude audible components ofthe facsimile signal from the speaker of the audio signal receiverprovided that the deviation produced by the carrier of the facsimilesignals is limited to about 5 kc. At 100 percent modulation, a facsimiledeviation of 5 kc., or a total swing of 10 kc., has been found toprovide very satisfactory reception of the sound and facsimile programs.The reason why this is so will be explained later in connection withFig. 6.

Extensive experimentation has demonstrated that good facsimile recordingmay be made with a maximum deviation of the radio frequency carrierproduced by the facsim-ile program, of only 40() cycles. However, withsuch an extremely small deviation, unusual care is necessary to preventinterference with the facsimile signals by the primary audio signals sothat a maximum deviation of at least a few kilocycles is to bepreferred. If the maximum facsimile deviation is increased much beyond10 kc., interference from components of the facsimile signal issometimes audible in the loudspeaker reproducing the primary audioprogram. This is one of the dimculties experienced in systems such asshown by Roder and Armstrong above mentioned.

If the transmitter is adjusted so that at 100 percent modulation of theradio frequency carrier by the audio frequency signals in the primarychannel, the carrier deviation is about 75 kc., and at 100 percentmodulation of the radio frequency carrier by the signals in theauxiliary channel, about 5 kc. deviation of the radio frequency carrieris caused, it is theoretically possible to have a total deviation of 8Okc. In practice however, such total deviation rarely occurs because fullmodulation of both primary and auxiliary signals very infrequentlyoccurs at the same instant. Even the theoretical possibility of such aminor trespass into the standard two 25 kc. guard bands (established bychannel separation of 200 kc. and permissible deviation of 75 kc.) maybe avoided by adjusting the primary signals to provide a maximumdeviation of 70 kc. This entails a loss of maximum reproducible soundvolume of less than one decibel, which is negligible and which can becompletely offset in practice by a trifling increase in transmitterpower.

The above description essentially describes the multi- Vquency modulated'radio frequency transmitter, 24 may be of conventional rtype suchas'shown onpage 205 'of Frequencj/ Modulationby A. Hund,.jpublishedinV1942 by lMc'GraWLI-Iill IBook Company, VNews York. In this typeof'tr-ansmitteritliemodulating signalsi'fr'om the'rn'ulti-V plexermaybefed'directlyjto the input transformer of the reactancemodulator ofthe transmitter. VInstead of supplying -mod-ulatedfsignals Afromthesignal 'source consisting-of facsimile scanner 7,'oscillator-8 andmodulator 9, a source of unmodnl-ated audio Vfrequency signals 6 maybeconnected "to 'the'preampliiier 10.' These audio frequencyjsignals maycover a maximum rangeof 0-10 kc., if desired 'andqmay be facsimile,telephone, 'telegraph, Teletype, or any `"other desired "type 'of audiofrequency signals.

In the'circuit of Fig. 3, vis shown a two channelimulti-Y plexerutilizing circuits in conformance with the block diagram 4of Fig. l.Channel l is the primary'channel provided to transmit audio frequencysignals in the range Vof litil-15,000 cycles per second and channel 2 isthe auxiliary channel which Ytransmits audio frequency signals in therangeof -'10,'000 C. P. S. originating as facsimile, voice,audioftelephone, telegraph, Teletype, etc. Additional auxiliary channelsmay be provided substantially identical with channel 2 to multiplexsignals from more thantwo sources as will be subsequently explained.

Inchannel -1 terminals 50, V51V are 'the'inputterminals of a transformer52Y having a primary winding53 and secondary Winding 54, a pair ofvariable YresistorsfSS, 56 balanced'to ground are connected Vto theterminals of the'seconda'ry winding Whose midpoint is also'grounded.Resistors 55, "56 are connected to a preemphasis circuit 5. Resistor 55isY connected directly Ytogrid 57 ofamplier'tu'be'SS. Resistor 56 isconnected togrid V5,9. of ampliiier'tube't). yCathodes 61,;62 of tubes58, 60 are connected together and grounded via resistor l63. Capacitor64 `by-passes highY yfrequency currents from the cathodes 61, 62 Vtoground. The tubes 58 and 60j are arranged as push-pull ampliers toeffect cancellation of even harmonics inthe output circuits of thetubes. The output-circuits of the tubes 58 and 60 are identical andconsist of load resistors 65, 66 connected via dropping resistor-67 to asource of positive voltage indicated as B+. YResistors 65, '66 areconnected to plates68, 69 of theamplier tubes. Theaudiofrequency'outputs of the tubes V58, V60 arel passed through Vcouplingcapacitors 70, 71 and resistor'sf72, 73 to a preernphasis network.

The preemphasis network consists of la series circuit bal- Y anced togroundincluding resistors 7 4, 75 and induc'tances V76, 77 inV parallelwith capacitors 7S, 79.Y The preemphasis network is designed to favoro-r emphasize the higher frequency components of the audio frequencysignals transmitted in accordance with accepted communicationsstandards. The balanced output of the preemphasis network Vis fed togrids V80, S1 of the push-pull amplifier tubes 82, 83.

Y Cathodes 84, V$55 are'balanced to ground Via resistors 86, 87. Theoutputs of the tubes are fed vvia platesfSS, 89 Vand couplingvcapacitors 90, 91 to the grids 92, V93 ofjthe push-pull amplifiertubesl 94, 95. VThe outputs of tubes 82, 3 Yinclude the equal loadingresistors 96, 97 joined together Vand Vconnecterdto the Vsource 'ofpositive V'voltage'B-k. The outputs of tubes'94, 95 are connected Vin aVnegative feedback arrangement with tubes 82,. 33. The 'feedbackarrangement includes the plates 98, 99 of tubes 94, 95 connected backtothe athodes 84,85 -of'tubess'via capacitors 10u-'w1 and resistant-e2,ies.' The feedback arrangement-reducesharmonic distortion'and improvesthe linearity of amplicationoftubes 94,95. AThe cathodes 1'04,- V105Aof' YtubesY 94, M95r are balancedto 'ground `via getherv constitute theamplifierV 21 as 'shown bythe,dotted lines. The plates of tubes 94, areconnected toithe, primary winding ofY a coupling 'transformer i111.v Theprimary winding 110 is tapped at its midpointand connected to the sourceof positive voltage B'-l-viare'` sistor l67 to provide plates 98, 99with 'the necessary positive potential. Secondary winding v112 Voftrans-VV former 111'is connected ,to the low `pass lter 23-which passessignais only up to about 16,000 cycles per second. The filter consistsofV a series arrangement of'inductances 113, 114, 115,116 and 117 in4parallel with capacitorsV 11.8, *119,* 120, 121. Capacitors 122, 123,124 shuntinductances 113,115, 117 respectively. The commenterminal ofcapacitors 118-121 is grounded. The output of the lter isA connected'viaa linear transmission network to the input terminals 125, 126 of'afrequency The linear the output'of'the auxiliary channel 2 which willnowbeV described.

In channelZ, transformer 133 Ahas a primary winding", 136 provided withthe input-terminals 134, 135. Sec-` ondary V137" yof the transformer hasa variable loadingV resistor 139 and a tuning capacitor 138. Thesecondary circuit is broadly'tuned'to pass a double sideband amplitudemodulated audio frequency subcarrien put of the secondary circuit istaken o the variable resistor 139 connected to grid 140y of theamplifier tube..

132 located in preamplifier 10. Cathode 141 is connected to ground viathe usual resistor'142 and'fbyhpass capacitor 14-3. The plate Y144 ofamplifier Vtube Y10is,v

connected via'coupling capacitor 145 Vto theV grid k147 'of cathodefollower tube 146.Y The cathode -210 of tube*` 146 is connected viacapacitor 214 to the'input trans!V former 14S of the ring or balancedmodulator 13, atthe midpoint of Vsecondary winding Y211. The midpoint ofprimarywinding 212 of the output transformer 149 is grounded. Asshownrin Fig. 3 the ring modulator 173` includes the input transformer14S, outputtransformer Y 149 and rectiiiers 1570-153 in Va balancedarrangement.

An oscillator 12 is provided as a'source of va subcarrier signal ofVsuperaudible frequency. The oscillator includes a resistor-capacitorassembly consisting of vfour 45 `degree sections containing resistors15S- 161 and capacitors 154-457. The resistor-capacitor assembly is,

connected to the control grid 163 of oscillator tube 164 via resistor162. The cathode 165 of tubeV 164 is con-V nected to ground via cathoderesistor 166 and by-pass capacitor 167. The oscillator output is fedback to the oscillator tube 164 and impresses these oscillations lon theprimary winding 174 of transformer 148 via resistorV 175. The Vplatecircuit of the Vcathode follower tube Y includes plate 176 and'resistbr177. By-pas's capacitor 178 is connected between plate 176 Vand ground.The secondary Winding 179 of transformer 149 issconnected to a lowrpassiilter 14 consisting of capacitorsr180f184 and inductors 135, 186, 137.The'output of'the iilterV is coupled via .shunt resistor 189fand'seriescapacitor 1x90 Y Y to the amplifier -15l consisting of the cascadedresistance coupled amplifier tubes 191,192. Y, An amplierf16, which maybe a cathode followenmaybe coupled to theoutput of amplilier'tube 192via capacitor '193 Yto operate meter'V resistors V106, 107 v'which yareI provided with *the by-pass capacitors 108,109. VTubes 82, 83, 94495101 The' The out# .of two stages of resistance coupled amplifiers.

asiopsa 17. The plate 194 of tube 192 is connected to thev primarywinding 195 of the coupling transformer 196. The secondary or outputwinding 213 of the transformer 196 is connected to the bandpass filter18, consisting of capacitors 197-205 and inductances 206-209. The outputof the filter 18 is connected via a linear attenuator 19 to the linearmixing pad 20 which is in turn connected to the terminals 125, 126 of afrequency modulator. The two channel circuit above described is apractical embodiment of that portion of the system shown as a blockdiagram in Fig. 1, which serves to multiplex the several audio frequencysignals according to the invention.

In operation of the multiplexer circuit of Fig. 3, audio frequencysignals in the range of 30 to 15,000 cycles per second are applied toterminals t), 51 of the primary channel 1. The signals are amplified intubes 58, 60 of the first balanced or push-pull stage and are thenapplied to the preemphasis network formed by elements 74-79. Thepreemphasis network imparts high frequency preemphasis to correspond tothe standard 75 microsecond curve specified by' the FederalCommunications Commission. If linear amplification is required omissionof inductances 76, 77 and capacitors 78, 79 will convert the preemphasisnetwork to a linear network assembly. The preemphasized signals are thenamplified in two succeeding stages of push-pull amplification. iscoupled by transformer 111 which has a linear transmissioncharacteristic to the low pass filter 23. This filter is designed topass the 30-15,000 cycles per second audio frequency signals and excludea'll frequencies, if any, higher than 16,000 C. P. S. The output of theprimary channel which is the output of filter 23 is combined in thelinear mixer pad 20 with the superaudible frequencyl signals from theauxiliary channel 2.

In auxiliary channel 2 of the multiplexer circuit, audio frequencysignals which preferably occupy a range of 7 to 13 kc. obtained byamplitude modulating a 10 kc. subcarrier with audio frequency signals ofO-3,000 C. P. S., aire supplied tothe channel input terminals 134, 135.As previously mentioned the 0-3,000 C. P. S. signals may originate assound signals, or as facsimile, telegraph, Teletype or other signals.-The modulated subcarrie'r signals are applied to a pre-amplifier stage10 via coupling transformer 133 and attenuator 139. The amplifier tube132 is resistance coupled to tube 146 operated as a cathode follower toprovide a low impedance driving source. The output of the preamplier 10is applied between the midpoints of the secondary winding of transformer148 and the primary Winding of transformer 149 at opposite sides of thebalanced modulator 13.

Oscillator 12 includes the oscillator tube 164 which in association withthe 180 degree phase reversing network including elements 154-161generates a 35 kc. carrier. This carrier is applied via the cathodefollower tube to primary winding of the transformer 148. The output ofthe modulator 13 is an amplitude modulated double sideband wave withcarrier suppressed because of the balanced structure of the ringmodulator. The modulator output which is applied to the low pass wavefilter 14 consists of two bands which are the heterodyne sum anddifference frequencies of the 35 kc. carrier with the 7 to 13 kc.modulated subcanrier. 'Ihe two bands occupy 22-28 kc. and 42 to 48 kc.,25 kc. and 45 kc. being the center frequencies respectively of the twobands. The higher 42 to 48 kc. band is removed by the filter 14. Theremaining 22-28 kc. band which passes the filter is amplified inamplifier 15. Amplifier 15 consists The output of the amplifier tube 192is passed through the linear coupling transformer 196 to bandpass filter18. Bandpass lter 18 passes on'ly signals in the range of 20-30 kc. sothat any harmonics or other frequency components, if any, other thanthose in the desired 22-28 kc. signal band are removed. The output ofthe auxiliary channel The final stage.

2 now consists .of a band of signals of superaudbie fre-` quency. Thesesignals are transmitted through attenuator 19 which is fixed so that themaximum amplitude of the signals is a predetermined value with respectto the maximum amplitude of the audio frequency signal output of theprimary channel 1. The meter 17 fed via amplifier 16 facilitates makingthis optimum amplitude setting. The reasons for this critical adjustmentof the amplitude will be discussed below in connection with Fig. 6. Theoutput of the auxiliary channel 2 is combined in the linear mixer pad 20with the output of the primary channel l and then delivered directly tothe frequency modulator of a radio frequency carrier via terminals 125,126. It will be noted that the common path for the audio frequency(3G-15,000 C. P. S.) signals and the superaudible frequency (22-28 kc.)signalls is a direct linear transmission network from the output offilters 18, 23 to the frequency modulator terminals 125, 126. Thepresence of the lters 18 and 23 insures that no undesired harmonics ofthe several amplifier outputs are present to be combined in the mixerpad. The pres ence of the linear transmission network 20 insures that nocross modulation can occur between the audio frequency and superaudiblefrequency signals between the several channelsv of the multiplexercircuit rand the frequency modulator of the radio frequency carrier. Nocircuit details are shown of the radio frequency transmitter andfrequency modulator 24 of Fig. 1, since this apparatus is entirelyconventional as heretofore mentioned and forms no part of thisinvention. The only require-l ment of the transmitter essential to thepresent invention is that the rradio frequency modulation be driven bythe output of the multiplexer circuit without any non-linear elementsuch as an amplifier between the multiplexer and modulator.

In Fig. 2, the members 25-33 constitute a conven-` tional frequencymodulation receiver such as is shown on page 753 of Radio Engineering byF. E. Terman, 3rd edition, published 1947 by McGraw-Hill Book Company,New York. Frequency modulated radio signals are `received on antenna 25and pass with various conventional translations, through R. F. amplifier26, converter 27, I. F. amplifier 28, limiter 29, discriminator 30,deemphasis circuit 31, and audio amplifier 32 to loudspeaker 33. Thisfrequency modulation receiver is operated in its normal mannerregardless of whether the auxiliary receiver channel to be described, isin operation or not. If the auxiliary receiver channel is in operation,.the radio frequency amplifier will amplify the modulated. carrierreceived by antenna 25. This carrier has a pre-V determined centerfrequency and is deviated at this frcquency at audio frequency andsuperaudible frequency rates. The instantaneous total swing isproportional to the combined instantaneous amplitudes of the originalmodulating signals. The converter 27 serves to heterodyne the modulatedcarrier down to a predetermined intermediate frequency range. The outputof converter 27 is a signal of intermediate frequency modulated at audiofrequency and superaudible frequency rates which is amplified in I. F.amplifier 28 and limited in amplitude to a predetermined value inlimiter 29. The input to discriminator 30 is an amplified intermediatefrequency signal limited to a predetermined maximum amplitude anddeviating at audio frequency and superaudible frequency rates. Themaximum deviation of the signal is of the order of 75 kc. Thediscriminator serves to develop a voltage output proportional inamplitude to the deviation in frequency of the signal input. The outputof the discriminator has audio frequency and superaudible frequencycomponents. The superaudible frequency components are attenuated in thedeemphasis circuit leaving the audio frequency signals which passthrough amplitier 32 and speaker 33 and are reproduced as the originalprimary audio program. The output of the dis- Y:z2-sierraseliminator-'30' 'is 'also delivered to Hthe auxiliaryw*receiverchanaelviasondwforjTheauxiliary y*receiyer 'channel "consists'of members 36 t'o 45., For purposes 'of illustrationofthefinventionjbut not *in limitation thereof the auxiliary/*channel isshown adapted for receiving and recording facsimile signals.

The :output of the 'discriniinator' 30"Vinclu'des'audio fre- Y quencysignais'dueito'th'e audiopro'gram 'having frequency" components 'in "therange 'of '3U-15:00@ CQP. S.,"ai rclV amplitude m'odulatedsignals ofsuperaudibleffrequeney inthe range( of 22 to'28 Vkcfca'rryingthe'anxiliarysignal Y program. 'These signals are'fapp'lied throughwireV 35V scanned `at the :transmitter station.

n"When .the Vtransmitter and 'receiver vstations of Figs. V1A and 2V'are operated in accordance with the teaching of this invention, thereis no perceptible sound in speaker 33 .due tothe auxiliary signalportion of the'signal received and there is no recording on paper '48due to the primary audio frequency signal portion of the signalreceived. ment of non-interference betweenthejseveral signals might bemet by'inserting a lowpass lter in the receiver ahead o'f deemphasiscircuit 31 to je'xclude all but 'audio frequency sound signalsfrom.speaker 33. However, Ythis expedient would beY commercially andpractically, impossible in 'view of the rights of the'many persons who'have frequency modulation audio receivers and are solely interested inreceiving 'frequency modulation audio programs. FurthermoreV such lowpass lters wouldY not prevent any audio frequency demodulation productsrepresenting 'the audio frequency auxiliary signals modulatedpon thesuperaudible carrier, from reaching'the audio ampliiier 32 and speaker33. YThese undesired audiofrequency demodulation products vare notremoved Vaccording to the present invention but ntheir amplitude is solowthat'they fall below the normal Itherinal noise level inthe audioamplifier 32; Their magnitude with re'spectto the Aprimary audio signalsis'at least`6'0 decibels below the primary audio signals sothatftheirrpresence is imperceptible'to an auditor of the-sound issuingfrom speaker '33. l Y Y For an 4explanation of thetheory underlying thesuc Y cess-cf the present invention'in eliminatin'g'interference fHowever, the signals which consist vof a' 22-28 kc, subcarrier amplitudemodulated' by frequencies Vin the range of 040,000 C. P. S. inpassingthrough discriminator 30 and audioV ampliiier 32 are'partiallydemodulated by inherent'non-linear` characteristicsY thereof and theresulting 9410,60() C.` P. S. fluctuations arein a range .of clearlyaudible sound. The. magnitudeof the sound in the'ispeaker dueto thepartially demodulatedpauxiliary,signals must he related to the,magnitudeof the Y,sound program re- It has been suggested that thisVessential"re'quirec Yiz c iterfrencelatall. VA'manner-in which'theauxiliary signal Iinterference-is*limited:to 60 decibels below thelsound isignalwill now be described..

I The V'denrodulation 'electjondistortion in discriminator iliary`signal modulation so that demodulation fin thev discriminato'rn'iaytbe` ignored. The 22-28 kc. Yauxiliary signals.infgoingthroughjaconventionaldeemphasis cir-A cuit Q31 Aarefa'ttenuated :by about -20decibels. A 3com' 1'()` ventional jaudiojampilifierf-32 Aintroduces Aademodulation distortion in the order'ofjS pereent.` This` degree ofdis'-Y Y tortion in Athe `amplilie'r 'meansetha't its 'output `carriesan.

audible auxiliary signal which'ris 26 decibels belowthe i amplitudeofthe 22-728 kc. 'subcarrier signal. Ifthe radio.

frequency carrier is frequency modulated to la'max'inium` deviation of70 kc. by the prima-ry audiofrequencyisig nals and 5 kc. bysuperaudible'ffrequency'subcarrietr of the auxiliary signals, thevauxiliary signals are initiallyV at least 2O decibels vbelowthe soundVsignals.: rifherefore, the audible Vauxiliary signals Vin speaker '33are at least 66 decibels below the'sound signal, this tigurebeingarrived at by adding the 20 decibels Ydifference between the primary andauxiliary signal energies in the received radio signal, the 420 decibels`attenuation of the .-su'peraudibleV frequency signals in VdeemphasisIcircuit 31 and the 26 decibels loss an auxiliarysignalstrengtlrresulting from 5 percent demodulation in vaudio amplifier 32.AllA interfe-ring signal in a'loudspeaker Whichis `'at least 6.6

decibels down from the desired sound signal is impossible of `detectionby the human ear. These nconclusionshave Y.

been born'out'byactual tests fof multiplexing 'equipment constructedaccording to the teaching ofthis invention. fn fFig. 6 isshowngraphically'the interference ratioin decibels asa measure of theaudible interference-prduced in a conventional frequency modulationreceiver as shown inFig. 2 byan amplitude modulated Asuper-` audiblefrequency subcarrier multiplexed with Y* Vaudio frequency signalsaccordingV to` the system ofFig. A1. The curve in.Fig.r6 is plotted'fora receiver having inher- Y figure of an Yaverage quality frequencylmodulationre-` ceiver. The total standard R. F. carrier Vdeviation Al"4ently 5% demodulation distortion which is the distortion is 75 kc. andthe deviations AF of the R.,F. carriergpro produced in consideringwhether auxiliary .signalinter-g ference is audible in ,the loudspeaker.An interference,

such .as is. present herein, whichis at least 60 decibels .below thesound signal isV universally considered as 'no duced by the amplitudeYmodulated superaudible frequency subcarriers are taken over the rangeof about'().5kc.1to'V 3l) kc. The 60 decibelslevel is taken as that inwhich`Y nozaudible interference willbe perceptible in the speaker 33.As'shown `in Frigf6, atJSF' equal'to approximately lOvkc.dev'iationjtheY interference ratio is at least minus' 60 dbs/*For alldeviations less than 'I'O'kc the -`interference ratio becomesprogressively steadily higher. Fig.

6, Villustrates that deviations up toV about l0 kc. produce( Fig. 2.YThecircuit includes a bandpass filter portion ,and ampliier Ysection37.The input terminals 250,*` 251 of the adapteramplierare to `be connecteddirectly toV the output of discriminator 3010i a conventionalfrequencymodulation vreceiver `as shown in Fig.V 2. No vamplifier may be usedbetweenfthe discriminator and filter V37 orobjectionable,intermodulation of the signals in the-dis-V criminatoroutput will {occur. series arrangement of capacitors '252-455, shuntedcapacitors 256, '257, 258, and shunted radiofrequency-chok coils 259,260, 261. Thelter is designedto pass signals in ythev rangegof 22428kc;` andV Y,sharply attenuate'gsignals outside of the bandpass region.Thek output of the 'ilt'erY is coupled via resistor 262'to .grid'263 ofamplilieri458."`

Cathode'265is connected to` ground via re'sistor`266 and bypasscapaci-tor' 2'67..y plate '264 of ampliiier 2'68Qis' coupled to VtgridV1699i .the r4cathtxie Yfollower 270 via coupling capacitor 271. A loadresistor 272 is provided Stl'is veryjs'mallevenfor relativelylargedeviation1aun Y still produce ,tolerableintefference The filterconsists ofay asioffse between plate 268 and a source of positivepotential B+. Bypass capacitor 273 is provided between B+ and ground.Plate 274 of the cathode follower is connected directly to the source ofpositive potential B+. Resistors 275, 276, 277 are provided in the gridcircuit of the cathode follower. The output of the cathode follower istaken off between cathode 278 and ground to be transmitted via terminals279, 289 to an amplifier such as the amplifier 38 of the recorderamplifier shown in Fig. 2.

An important modification of the invention will now be describedparticularly with reference to a multiplex system including more thantwo channels as shown in Fig. 5. The system includes a primary audiochannel and four auxiliary signal channels. The primary audio channeltransmits the usual broadcast audio frequency signals covering7 the fullrange of 30-l5,000 cycles per second. The primary audio channel isidentical with that of Figs. 1 and 3. The amplifier 21 provides anoutput of predetermined maximum amplitude to cause a predetermineddeviation of the R. F. carrier in the frequency modulation of the radiofrequency transmitter. The several auxiliary channels have anarrangement corresponding to that shown in Figs. l and 3. Oscillators 12generate subcarriers of different superaudible frequencies which aremodulated in the respective balanced modulators 13 by the amplifiedsignal outputs of amplifiers 10. The modulated subcarrier is amplifiedin an amplifier 15 and then passed to a bandpass filter 18. A lineartransmission network follows each bandpass filter and consists ofattenuators 19 and mixing pad 20. From attenuators 19 the outputs of allchannels are combined in the linear mixing pad and then fed to thefrequency modulator 24 of the R. F. transmitter to modulate the radiofrequency carrier generated therein. Each of the attenuators 19 is setto provide a modulated superaudible frequency signal of predeterminedmaximum amplitude so that a predetermined deviation of the R. F. carrieris produced by the signal output of each auxiliary channel. Thefrequency modulated R. F. carrier is then radiated by the antenna 22 tosuitable receiving systems to be described. p

The receiving systems provided for the several signals multiplexed bythe system of Fig. will of course depend on the type of signals carried,whether aural, voice audio, facsimile, telegraph, Teletype or otherwise.In general a standard frequency modulation receiver will sutlice for theprimary audio program channel as shown by members 25 through 33 of Fig.2. A typical receiving station for plural auxiliary channels is shown inFig. 7. The members 25 through 30 are common to all receiving channels.Members 36 through 46 in one channel are arranged to reproduce graphiccopy from facsimile signals as also shown in Fig. 2. Members 36, 37, 39,32', 33 in the other channel are arranged to reproduce auxiliary signalsof another type such as aural, voice audio, Teletype, telegraph, etc. Asmany receiving channels as required similar to those shown in Fig. 7 maybe provided to receive signals from all channels multiplexed by theapparatus of Fig. 5. The bandpass filters 36 at the input of eachreceiving channel select the particular band of modulated superaudiblefrequency signals to be demodulated in that channel.

The multiplex system of Fig. 5 requires that careful consideration begiven to the respective deviations of the R. F. carrier to be producedby the primary and several auxiliary channels. The maximum possibledeviation is required for the primary audio frequency channel to insurea maximum signal-to-noise ratio therein. The auxiliary channels may notbe assigned too small an R. F. deviation however, or the signal-to-noiseratio in these channels will be so high that satisfactory reception canonly be obtained in a service area which is relatively small comparedwith the normal services area of the primary audio channel. In generalthe higher the deviation assigned to each multiplexed channel, thehigher will be the signal-to-noise ratio at any particular receivinglocation channel.

14 for the several channels. In the duplex system disclosed heretofore,deviations of the R. F. carrier up to ten kilocycles produced by theauxiliary channel were discovered to provide optimum interference ratiosfor the primary It was further found that a deviation AF as small asfive kilocycles extended the service area of the single auxiliarychannel to an area approximating the normal service area of the primarychannel.

In order to provide a multiplex system as shown in Fig. 5 including fourauxiliary channels in addition to the primary channel for a standardsystem employing a total R. F. deviation AF of 75 kc. a deviation AF offive kilocycles may be provided for each auxiliary channel. In such anarrangement a total of 20 kilocycles may be assigned to four auxiliarychannels, leaving 55 kilocycles for the deviation produced by theprimary audio program channel. In such an arrangement a slight reductionin the signal-to-noise ratio of the primary audio program is caused butthe original signal-to-noise ratio (of the transmitter operatingsimplex) may be recovered by a relatively small increase in theeffective radiated power of the transmitter. If the area of coverage ofthe four multiplexed auxiliary channels may be somewhat limited, thatis, if the area may be materially less than the normal service area ofthe transmitter for the primary audio program, a deviation AF ofconsiderably less than ve kilocycles per auxiliary channel may beassigned and satisfactory multiplex communication according to theinvention will be obtained. Thus if 2.5 kilocycles of deviation areassigned to each of four auxiliary channels, the total AF deviationproduced by all the auxiliary channels will be ten kilocycles. Thisdeviation will produce an interference ratio as shown in Fig. 6 of morethan 60 db in receivers having a maximum of 5% of modulation distortion.The tinal determination of the deviation to be assigned to each channelof a multiplex system according to the invention will thus depend onfactors such asthe number of channels in the system, prescribed minimumsignal-to-noise ratio for each channel, tolerable inter-channelinterference, service area required for each channel, distortion in thereceivers of the several channels, etc.

In the system of Fig. 5 of each of the auxiliary channels transmitsaudio frequency signals which may be in the range of 0-5 kc. Oseillators12 may generate 35, 45, 55, and kc. subcarriers respectively. Theoutputs of modulators 13 are double sideband amplitude modulated signalswith suppressed carriers. The modulated subcarriers occupy the frequencybands of 30-40, 40-50, 50-60 and 60-70 kc. respectively. These modulatedsubcarriers are amplified in the amplifiers 15 and then fed to thebandpass filters 1S which pass only the respective frequency bandsindicated to the linear transmission network. The arrangement of Fig. 5has been described for double sideband wave transmission in eachchannel; The same arrangement can be used for vestigial or singlesideband wave transmission which will permit a broader band of audiofrequency signals to be transmitted in each auxiliary channel. Thus ifthe auxiliary audio frequency signals cover a range of 0-10 kc. and theoscillators 12 generate 39, 49, 59, and 69 kc. subcarriers respectively.The double sideband outputs of the modulators 13 will be 30-49, 40-59,50-69 and 60-79 kc. respectively. The bandpass lters will pass only thelower sidebands 30-39, 40-49, 50-59 and 60-69 kc. respectively if thefilters 18 are designed to cut olf the signals between 39 and 40, 49 and50, 59 and 60, 69 and 70 kc. respectively.

The system of Fig. 5 is also adapted for frequency modulation of thesuperaudible frequency subcarriers. Thus the audio frequency signals mayfrequency modulate the superaudible frequency carriers and then themodulated subcarriers pass through amplifiers 15 and band pass filters18 to the linear transmission network. The maximum amplitudes of themodulated subcarriers will be set at predetermined values with respectto the amplitude Yof n the Aprimary audio channel `/outpntto-iix themaximum deviations of the R. F. 'carrier' produced by each channeloutput as such relative amounts that cross interference between thereceived signals is minimized. For frequency modulation of thesuperaudilblle frequency subcarriers in Figs. l or V by audio'frequencysignals up to 5 kc. a deviation of each `subcarrier of 5 kc. will beproduced, for a deviation ratio of unity.

Y If a deviation ratio greater than unity is desired such as 2,

for example, then the 5 kc. deviation may be retained but the maximumfrequency ofthe audio frequency signals will be 2.5 kc. The deviationratio mentioned is,V of course, the ratio of carrier deviation tomaximum modulating frequency.

It will be noted that the Alow pass filters 14 employed in i the systemsof Figs. l and 3 maybe omitted from Fig. 5.

These'lters are shown in 'dotted lines to indicate Vthat they may beusedif desired Vto insure that the signals fed Vto amplifiers areabsolutely free of frequency components higher than the maximumfrequency to be transmitted by the band pass filters 18. VIn no eventmay lters 18 be omitted. Each ofl attenuators 19in Fig. 5 .is set toprovide a predetermined maximum amplitude for each channel output sothat the interference ratioV of interfering signals reproduced inreceivers arranged as shown Y in Figs. 2 and 7 is at least 60 db. Inthis way no perceptible interference between the Aseveral multiplexedsignals is detectable in the outputs ofthe receivers V'of all channels.t

Y Inthe description of the invention certain of the superaudiblefrequency subcarriers in the auxiliary .channels have been described asbeing Vmodulated by audio frequency `subcarriers whichl are themselvesmodulated byY the several auxiliary ,audio frequency signaling currents.It is of course quite possible as illustarted in Fig. 5 to omit theaudio frequency subcarriers for certain types of signals and directlymodulate the superaudible 'free quency subcarriers by unmodulat'edkaudio frequency signalingpcurrents. Y It is also considered as comingwithin the Vscope Yof the ,present invention to frequency modu-V late,phase modulate,Y or'modulate in Yotherflmown ways,

the severalfsuperaudible frequency subcarriers rather than amplitude.modulate them jand to provide appropriate demodulators in the auxiliaryreceivingchannels to demodulate `the modulated subcarriers to reproducethe original auxiliary signaling currents. Y Y,

' While'theinve'ntion has been described in considerable 'for'. passingonly amplified audio frequency signals; a plurality of auxiliaryVchannels, each including a sor'ce of otheraudio frequency signals and asource of signals of predetermined superaudible frequency connected'topamodulator toj produce amplitude modulated double sideband Ysupera'udiblefrequency signals, saidm'odulator'beingY connected toV a'filter forpassing only one of the sidebands, an

amplifier for kthe passedsideband, and .arlinal lter rfor passing onlythe'amplied sideband, Ythe sup'eraudible sig*- rials in each auxiliarychannelioccupying a different,prede'-V termined frequencyl range so thatthe'several sideban'cls passingthe said linal'lters cover brandsV ofdifferent super Y audiblejfrequency range, each of saidtnal'fflltersbeing connected to attenuating means ifornxingth'e maximum amplitudes of the signals ,passingLthe terminating andinal V"filters Vatpredetermined relative'value's, and a transmission Vdetail and byreferencerto specific frequencies', it willrbie means including va padconnected directly tosaid attenuating means and Vterminating filterandiprovidingca commonline'arfpath for 'the' filtered audio frequencysig'- Y nals and thelltered bandsof sup'eraudible'frequency signalsdirectly from the' several channels to'input terminal of Vafrequencyrnodul'ator for said carrier. c

2. Amnl'tiplex system includingi'means for frequency modulatingaradio`frequency carrier, `comprising at least two jchannelsyone of saidchannels includingV in circuitV in succession 'a 4source of 'first audiofrequency signals, an amplifier, and a first lter for said signals; theother of said channels'including' in circuit inisuccession a `source'ofd'ouble sidebandfsuperandible frequency signals amplitudemodulatedother audio lfrequencysignals, another'lter for passingthe'lowerione Vof said sidebands, an amplifier and a bandpass filteiforsaidrlowcr sideband; and `a linear transmission `networkV providing Va'common path for filtered bands'of audio'frequency and'snperaudiblefrequency signals directly fiomsaid first filter and bandpass ilter're-Y spectively to terminals of a'r'frequency modulator for said radiofrequency carrier; said channels'in'cluding attenuating means forjtixingvthe maximum amplitudes of said ltered audio'frequency and superaudiblefrequencysignals at predetermined relativeivalues Vto 'minimize'interaference in an associated receiving system.`

Y 3. A multiplex system according to claim 2, wherein said source ofsuperaudible frequency signals 'comprises' a source ofsaid otheraudiofrequency signals and argenerator Yof'a Y'superau'dible frequencysubcarrier, connected toa balanccd'modulator.

i4. A"multiplex system'according to ,claim Y2, further comprising meansffor receiving and'jdemodulating said frequency modulated` Vcarrier to*reproduce `said filtered band of 's up-eraudilple vfrequency signals, aband-pass filter.

connected'to'saidmeansjto'rpass only the `reproduced band ofsuperaddiblefrequency signals, and' a demo'dulratorcon-u nected to thelast-manned filter to reproduce said'other audio frequency signals.VV YY 5; A Vnmltiplex `system 4accordingV tofclaim'VV 4, --where`inisaid'source of otherjaudio Vfrequency signalsc'omprisesra generator ofan, audio frequency subcarrier 'andfa source of auxiliary audid'requencysignals, 'connectedtoan am'- plitude modulator. Y YU M Y Y 6. Amultiplex system `according to'claim 5, `wherein said Vsource Vofauxiliary audio frequency signals is `a facsimile scanner.V

7..A,multiplerx systemjaccording to claim v6,'furtherAV comprising meansVforjreceiving and demodulating said Y frequency modulated carrier-"toreproduce said `filtered band of superaudible'frequency signals;ajbandpasrslter connectedto said means to'pass V,only the reproducedband of VsuperaudibleV frequency signals, a demodulator' connected tothe last4 named jfilter to reproduce -audio fre@ quency Yfacsimile'signals,fand means to translate'saidAV facsimile Ysignalsrinto graphiccopy;

8. A multiplexsystem according to claimn 2,' whereinl said `receivingYsystemcc-mprises means for Lreceiving and demodulatin'gsaid"frequencymodulated radio frequencyl carrier, and meansrfor;translatingthedomodulate'd-V carrier into sound -of andiblelamplitudes,said soundhaving-,interfering components at least 60'deci-bels lower'in amplitudethan the lowest amplitude of said'sound.

y9. A multiplex systemfncludrvg-means ofotherg'ofsa'i'dcharinels'fearliy including Vin circuit.A

session sourcefof` doublesideband sfuperaudiblefrequency Vsignalsamplitude-mndulate'd "by ,othery audio frequency sign-alsyanothe'r'filter for#passi-ng'Y the lower one of; said sidbands;van-ampliifiei"and a Vbandpass .lter `for Vpassing only A'said iov-zeisideband, each fof-the Y several `lower, side'. bandscoverin'gxa'-Ydilferertband Y of superaudible freque'nciers'rand i linear transmission.network providing a com-v ifor *frequency 1 modulatingfa radiovfrequrzncrf'"carrier, comprising ayplu# rality of channels; one-of saidchannels including in 'circuit' in succession -a Ysource. of` iirstaudiofrequency signals',V an ampliefrfand airst'filterifor'said Vsignalsgfapl 'ali v mon path for a ltered band of audio frequency signals andltered bands of superaudible frequency signals directly from the rstlter and bandpass filters respectively to terminals of a frequencymodulator for said carrier; said channels including attenuating meansfor fixing the maximum amplitudes of said filtered audio frequency andsuperaudible frequency signals at predetermined relative Values tominimize interference in an associated receiving system.

10. A multiplex system according to claim 9, further comprising meansfor receiving and demodulating said frequency modulated carrier toreproduce said 'filtered bands of superaudible frequency signals, aplurality of bandpass lters connected to said means to pass only pre-References Cited in the iile of this patent UNITED STATES PATENTS2,233,183 Roder Feb. 25, 1941 2,258,871 Wedig Oct. 14, 1941 2,578,714Martin Dec. 18, 1951 OTHER REFERENCES Termans Radio Engineering, 3rdedition, page 482.

